Oh crap. Can't you even put it in DFU mode ?
Hold power button four seconds, keep pressing it and press home button simultaneously for another 8 seconds until the glowing apple has disappeared since about half a sec, then release POWER button only holding home burin down for a last 8-ish seconds.
If screen stays black its in DFU mode.

Ok. Press both power and home buttons together until the apple appears, then let go of both buttons and press the volume up button instead. Does your phone boot up then ? This should bring it into safe mode.